In the case of developing an electrostatic latent image produced on a record sheet with the aid of developing solution containing colored pulverized powders suspended therein (hereinafter called toner particles), for example, the toner particles are electrically charged with a polarity which is reverse that of the electrostatic latent image so as to make use of the electrophoresis of the toner particles for the purpose of converting the electrostatic latent image to a visual image.
In order to accelerate the electrophoresis of the toner particles, that is, in order to effectively adhere the toner particles to the electrostatic latent image, it has been known to provide a counter electrode opposed to the electrostatic latent image surface. It has also been proposed to arrange this counter electrode at a position spaced apart from the electrostatic latent image surface by a distance of the order of 20 .mu. in order to adhere the toner particles to the electrostatic latent image in the most efficient manner. For this purpose, roller counter electrodes have generally been considered as the most efficient counter electrode.
Experimental tests have yielded the result that the use of roller electrodes whose number is so increased that the electrostatic latent image is sufficiently saturated with the toner particles ensures an improvement of various developing characteristics. However, the use of such large number of roller electrodes results in an increase of the number of driving mechanisms thereof, thereby rendering the developing device as a whole expensive. In order to eliminate such drawbacks, heretofore, it has been proposed to use a stationary electrode composed of a guide member for guiding the transfer of the record sheet in the developing solution and exhibiting an effect comparable with that of the roller electrode.
Such conventional stationary electrode is composed of an electrically conductive plate provided with a number of punched holes each having a diameter of the order of 1 mm and spaced apart from each other by a distance of the order of 2 mm or is composed of an electrically conductive plate provided with a number of strip-shaped grooves arranged transversely to the advancing direction of the record sheet. The former stationary electrode, however, has the disadvantage that electric lines of force become irregular and cause the visual image to produce a strip-pattern. In the latter stationary electrode, there is the risk that the record sheet can become clogged, and as a result, the latter stationary electrode is very inconvenient to be used as the guide member for guiding the transfer of the record sheet in the developing solution.